Method of manufacturing a light guide

ABSTRACT

A method of manufacturing light guide by filling a polymer tube with a monomeric mixture then pressurising and heating the full length of the polymer tube to initiate and maintain polymerisation in the tube. An apparatus for performing the method is also described in which the polymer tube is placed in a reaction vessel and a temperature controlled fluid is circulated to regulate the temperature in the vessel.

FIELD OF THE INVENTION

This invention relates to a method of manufacturing a polymer lightguide. In particular, it relates to a method that produces a light guidethat is essentially free from voids and gas bubbles.

BACKGROUND TO THE INVENTION

Production of light guides from polymeric materials is well known. Avariety of different techniques for their production may be found in theliterature.

Reference may be had to U.S. Pat. No. 4,422,719 in the name of Orcutt.Orcutt describes a flexible thermosetting polymer produced as a lightguide. The polymer composition is disclosed as 60-90% allyl diglycolcarbonate (CR39), a 1.5 to 3% solution of isopropylpercarbonate (IPP)and the balance methyl methacrylate. The monomer solution is polymerisedin a hot liquid bath for a period of 12 to 24 hours. Orcutt disclosesthat the core material may be under catalysed in order for it to remainflexible.

Reference may also be had to U.S. Pat. No. 5,298,327 in the name ofZarian. Zarian describes a plastic light conduit of cross-link polymermaterial having good light transmitting characteristics. Also disclosedis a method of manufacturing the light conduit involving progressiveheating of reactive monomer mixture in a tube which is submerged in aprogressive reactor utilizing cold oil to maintain a cold, non-reactingzone, and hot water to create a hot zone that causes reaction, whilesimultaneously carrying away the excess exothermic heat of reaction.

Although the Zarian method may produce polymer light guides of goodquality that are free from bubbles and voids it is a slow technique.Furthermore, the progressive nature of the method places a practicallimit on the length of light guide that can be produced due todifficulties in handling tubes filled with unreacted liquid monomer.

OBJECT OF THE INVENTION

It is an object of the present invention to provide a method ofmanufacturing a polymer light guide that substantially alleviates thedeficiencies in the prior known methods.

DISCLOSURE OF THE INVENTION

In one form, although it need not be the only or indeed the broadestform,.the invention resides in a method of manufacturing a polymer lightguide including the steps of:

producing a monomeric mixture from amounts of monomer, initiator andcross-linking agent;

forming a polymer tube into a coil;

filling the coil of polymer tube with the monomeric mixture;

pressurising and heating the full length of the polymer tube toconditions appropriate to initiate and maintain polymerisation of themixture.

In preference, the method further includes the step of stirring themonomeric mixture to form a uniform mixture. The mixture is preferablyallowed to degas for a period of time. The period of time is typicallyin the order of 30 minutes.

The method may further include the step of purification of the monomerbefore producing the monomeric mixture. Purification may be by vacuumdistillation, ion exchange filtration or other suitable process.

The method may also include the step of pre-expanding the polymer tubeby pressurising the tube prior to filling the tube with the monomericmixture.

The polymer tube is preferably greater than 100 metres in length. Morepreferably the polymer tube is greater than 300 metres in length. Inaddition it is preferable that the coil of polymer tubing is formed bywinding polymer tube onto a drum.

The step of pressurising and heating the full length of the polymer tubemay suitably be conducted in two or more steps in which the mixture inthe polymer tube is subjected to a first set of temperature and pressureconditions followed by second and further sets of temperature andpressure.

The conditions appropriate for initialisation of polymerisation includea pressure in the range 10 kpa to 2000 kpa.

The temperature is preferably maintained in a range between the storagetemperature of the mixture and a temperature whereby the half life ofthe initiator used is between 10 and 20 hours. The temperature of themixture is preferably increased to a value whereby the half life of theinitiator used, is between 5 and 10 hours. The temperature may befurther increased to a value whereby the half life of the initiator usedis between 30 minutes and 60 minutes to obtain a desired final cure.

In a further form the invention resides in an apparatus suitable formanufacturing a polymer light guide, said apparatus comprising,

a means for supporting a coil of polymer tube;

a reaction vessel for holding a polymer tube fillable with a monomericmixture;

pressure means connectable to the polymer tube for applying pressure tothe tube before and after filling said tube with said monomeric mixture;

a temperature controlled fluid, circulable through the reaction vesselso as to control the temperature within the reaction vessel;

a pump for circulating the temperature controlled fluid; and

temperature control means for adjusting the temperature of thetemperature controlled fluid.

In preference the polymer tube is completely immersed in the fluidwithin the reaction vessel. The reaction vessel may be a room. The meansof supporting the coil of polymer tube may be a drum.

The fluid may conveniently be water although other fluids, such as oil,will be necessary if high temperature initiators are used. Air may alsobe used as the fluid for temperature control. In the case of air, thepump is conveniently a fan. Air will be the fluid of choice when thereaction vessel is a room.

The apparatus may further include a filling means. The filling meanssuitably comprises a pressure vessel containing a reservoir of monomericmixture, a bleeding tank for receiving excess monomeric mixture andpressure supply means for supplying pressure to the pressure vessel toforce monomerix mixture through the polymer tube when connected betweenthe pressure vessel and the bleeding tank.

Polishing means may also be incorporated into the apparatus forpolishing the optical entrance and exit faces of the polymer lightguides produced in the apparatus.

BRIEF DETAILS OF THE DRAWINGS

To assist in understanding the invention preferred embodiments will nowbe described with reference to the following figures in which:

FIG. 1 is a graph of the polymerisation profile of a first light guide;

FIG. 2 is a graph of the polymerisation profile of a second light guide;

FIG. 3 is a graph of the polymerisation profile of a third light guide;

FIG. 4 is a graph of the polymerisation profile of a fourth light guide;

FIG. 5 is a graph of the polymerisation profile of a fifth light guide;

FIG. 6 is a diagram of a vertical reactor for working the method;

FIG. 7 is a diagram of a horizontal reactor for working the method;

FIG. 8 is a diagram of a ‘U’ shaped reactor for working the method;

FIG. 9 is a diagram of a double ended reactor for working the method;

FIG. 10 is a diagram of a reaction chamber for production of long coilsof light guide;

FIG. 11 is a diagram of a reaction chamber for production of long coilsof light guide on a drum; and

FIG. 12 is a diagram of a filling apparatus.

DETAILED DESCRIPTION OF THE DRAWINGS

The specific embodiments described herein are by way of example only anddo not define the full range of possible formulations falling within thescope of the invention. In the drawings, like reference numerals referto like parts.

A flexible light guide is formed by polymerising a mixture of monomers,multi functional cross-linking agents, UV stabilisers/absorbers andinitiators.

The monomer is an alkyl acrylate, alkyl methacrylate or any vinylmonomer, which may be selected from the following list (although notlimited to the compounds in the list).

methyl methacrylate

butyl methacrylate

decyl methacrylate

ethyl acrylate

butyl acrylate

2-ethylhexyl acrylate

styrene

The multifunctional cross-linking agent may be selected from thefollowing list (although not limited to the compounds in the list).

allyl diglycol carbonate (CR39)

diallyisophthalate

divinyl benzene

triethyl glycol diacrylate

ethylene diacrylate

diallyl carbonate

The initiators may be selected from the following list (although notlimited to the compounds in the list).

di-isopropyl peroxydicarbonate (IPPC)

dilauroyl peroxide (LP)

dibenzoyl peroxide (BPO)

tert-butyl peroxy 2-ethylhexanoate (TBPEH)

1,1-di((tert-butylperoxy)3,3,5 trimethyl cyclohexane) (TMCH)

dicumyl peroxide (DCUP)

di(tert-butyl)peroxide (DTBP)

di(2-ethyIhexyl)peroxydicarbonate (EHPC)

dicyclohexyl peroxydicarbonate (CHPC)

sec-butyl peroxydicarbonate (SBP)

n-propyl peroxydicarbonate (NBP)

isopropyl peroxide

methyl ethyl ketone peroxide

Ultraviolet stabilisers/absorbers may also be added to the monomericmixtures according to the method disclosed in our co-pending applicationnumber PP3321 titled IMPROVED LIGHT GUIDE.

The polymer tubes filled with monomeric mixture may be formed fromsuitable fluoropolymers selected from the following list (although notlimited to the list).

Poly-tetra-fluoro-ethylene (PTFE)

Poly-chloro-tri-fluoro-ethylene (PCTFE)

TFE and perfluorovinylether copolymer (PFA)

TFE and hexa-fluoro-propylene copolymer (FEP)

Poly-vinyliden-fluoride (PVDF/PVF2)

TFE and ethylene copolymer (ETFE)

CTFE and ethylene copolymer (ECTFE)

The polymer tubes may also be formed from non-fluoropolymers such aspolyethylene teraphthalate (PET).

The following five polymers were produced for testing purposes. Thepolymers were produced in an FT-IR machine prior to the preparation ofthe light guide in the reaction vessel so that polymerisation profilescould be obtained.

EXAMPLE 1

A mixture was formed comprising 50 parts by weight of distilled MMA, 1part by weight of BPO (dibenzoyl peroxide) and 50 parts by weight ofCR39. The mixture was stirred for 30 minutes with a magnetic stirrer toobtain a uniform mixture, then degassed for a further 30 minutes.

An FEP tube was immersed in a bath of water at 60° C. and the tubefilled with the liquid mixture. The tube was pressurised to 300 kpa.After 3 hours the temperature of the bath was increased to 75° C. andleft for a further 7 hours. At the end of this time the pressure wasreduced to ambient and the water bath was heated to 85° C. and left for30 minutes. The polymerisation profile is shown in FIG. 1.

EXAMPLE 2

A mixture was formed comprising 50 parts by weight of distilled MMA, 1part by weight of lauroyl peroxide and 50 parts by weight of CR39. Themixture was stirred for 30 minutes with a magnetic stirrer to obtain auniform mixture, then degassed for a further 30 minutes.

An FEP tube was immersed in a bath of water at 60° C. and the tubefilled with the liquid mixture. The tube was pressurised to 500 kpa.After 6 hours the pressure was reduced to ambient and the polymerisationwas left to continue for 12 hours. The polymerisation profile is shownin FIG. 2.

EXAMPLE 3

A mixture was formed comprising 50 parts by weight of distilled MMA, 1part by weight of BPO and 50 parts by weight of distilled butylmethacrylate. The mixture was stirred for 30 minutes with a magneticstirrer to obtain a uniform mixture, then degassed for a further 30minutes.

A PFA tube was immersed in a bath of water at 75° C. and the tube waspre-expanded at a pressure of 550 kpa for 60 minutes. The temperaturewas reduced to 60° C. and the pressure reduced to ambient. The tube wasthen filled with the liquid mixture and pressurised to 100 kpa. After 3hours the temperature of the bath was increased to 75° C. and left for afurther 1 hour. At the end of this time the pressure was reduced toambient and left for a further 30 minutes. The polymerisation profile isshown in FIG. 3.

EXAMPLE 4

A mixture was formed comprising 45 parts by weight of distilled MMA, 1part by weight of BPO, 5 parts by weight of distilled divinyl benzeneand 45 parts by weight of distilled 2EHA. The mixture was stirred for 30minutes with a magnetic stirrer to obtain a uniform mixture, thendegassed for a further 30 minutes.

An FEP tube was immersed in a bath of water at 75° C. and the tube waspre-expanded at a pressure of 500 kpa for 30 minutes. At the end of thistime the pressure was reduced to ambient and the water bath was allowedto cool to 60° C. The tube was filled with the liquid mixture. The tubewas then pressurised to 100 kpa and left for 200 minutes. Thepolymerisation profile is shown in FIG. 4.

EXAMPLE 5

A mixture was formed comprising 50 parts by weight of distilled vinylacetate, 1 part by weight of di(2-ethylhexyl)peroxydicarbonate and 50parts by weight of CR39. The mixture was stirred for 30 minutes with amagnetic stirrer to obtain a uniform mixture, then degassed for afurther 30 minutes.

An FEP tube was immersed in a bath of water at 75° C. and pre-expandedat a pressure of 500 kpa for 60 minutes. The temperature was reduced to50° C. and the pressure was reduced to ambient. The tube was then filledwith the liquid mixture, pressurised to 100 kpa and allowed to continuepolymerisation for 750 minutes. The polymerisation profile is shown inFIG. 5.

The following two procedures exemplify variations to the proceduresdescribed above fr/or producing polymer light guides in extendedlengths.

EXAMPLE 6

A mixture was formed comprising 80 parts by weight of distilled MMA, 1part by weight of BPO (dibenzoyl peroxide) and 20 parts by weight ofCR39. The mixture was stirred for 20 minutes with a magnetic stirrer toobtain a uniform mixture.

A 420 metre long FEP tube in coil form was immersed in a bath of waterat 60° C. and the tube filled with the liquid mixture using the fillingprocess described below with reference to FIG. 12. The tube waspressurised to 300 kpa. After 2 hours the temperature of the bath wasincreased to 70° C. and left for a further 2 hours. At the end of thistime the pressure was reduced to ambient and the product removed fromthe bath.

EXAMPLE 7

A mixture was formed comprising 50 parts by weight of distilled MMA, 1part by weight of DCUP (dicumyl peroxide) and 50 parts by weight ofCR39. The mixture was stirred for 30 minutes with a magnetic stirrer toobtain a uniform mixture, then degassed for a further 30 minutes.

A 150 metre long FEP tube was placed in coil form in a chamber in avertical drum at 80° C. and the tube filled with the liquid mixtureusing the filling apparatus described below with reference to FIG. 12.The tube was pressurised to 70 kpa. After 2 hours the temperature of thechamber was increased to 120° C. and left for a further 6 hours. At theend of this time the pressure was reduced to ambient and the productremoved from the chamber.

The polymer light guides resulting from the above examples were testedfor voids and bubbles. The tests showed that all the light guides wereessentially void and bubble free. But the light guide in Example 2 andExample 4 were slightly opaque.

An apparatus suitable for performing the method is shown schematicallyin FIG. 6. The apparatus comprises a vertical reaction vessel 1 in whicha fluoropolymer tube 2 is located. Hot water, or other fluid such as oilor air, is circulated from a temperature controlled reservoir 3, throughlines 4 by pump 5. The temperature of the circulating fluid determinesthe temperature within the reaction vessel 1.

The inventors have found that control of the temperature in the reactionvessel is easiest with the flowing arrangement described. However, atemperature controlled static bath could also be employed.

Pressure can be applied to the tube 2 by pressure supply means 6, whichmay be hydraulic or pneumatic. Pressure supply line 7 connects thepressure supply means 6 to the tube 2 via manifold 8 and coupling 9(seen in cut away portion 10). The tap 11 in the manifold 8 can isolatethe tube 2 from the pressure supply means 6. The tap 12 can then beopened to allow monomeric mixture to flow from reservoir 13 into thetube 2. The other end of tube 2 is conveniently closed by plug 14 asshown in cut away portion 15.

Although the apparatus is shown with a single tube 2 it will beappreciated that multiple tubes can be processed simultaneously byappropriate arrangement of the manifold 8. Furthermore, a number ofreactors can be operated together for mass manufacture of optical lightguides.

A further apparatus suitable for performing the method is shown in FIG.7. In this embodiment the reaction vessel 16 is a double-endedhorizontal reactor. Water is circulated from reservoir 3 by pump 5 as inthe first embodiment. In the embodiment of FIG. 7 pressure is applied tothe tube 2 from both ends. A manifold 8 is provided at both ends of thereactor thereby allowing monomeric mixture to be sucked into the tube 2from reservoir 13 by application of suction to line 17.

A support tube 18 may be fitted within the reaction vessel 16 as shownin cut away portion 19. The support tube 18 provides a limit to theexpansion of tube 2 when it is being pre-expanded or filled withmonomeric mixture. The support tube 18 reduces the possibility of thetube 2 splitting due to the pressure applied.

A still further reaction vessel 20 in the shape of a ‘U’ is shown inFIG. 8. FIG. 9 shows a double ended reaction vessel 21. The arrangementof temperature control, pressure control and filling of the tube for theembodiments of FIG. 8 and FIG. 9 are similar to the arrangementdescribed for the vertical 1 and horizontal 16 reaction vessels. Eachreaction vessel 1, 16, 20 and 21 is suitable for working the methoddescribed herein, however the double ended and ‘U’ shaped reactionvessels have particular advantage in compact design where longer lengthsof light guide are required. It will be appreciated that the variationsdescribed above can be applied to any of the specific reaction vesselembodiments or to other reaction vessel designs that will be evident topersons skilled in the relevant art.

Very long polymer light guides can be produced using the reactionchamber shown in FIG. 10. The reaction chamber 22 is a drum in which acoil of polymer tube 23 is placed. The coil may conveniently be wound ona wooden drum. A lid 24 is placed on the chamber 22 to assist withtemperature control and to protect against the effects of splitting ofthe tube when under pressure. The polymer tube 23 may be pre-filled withmonomeric mixture before being placed in the chamber 22. To facilitatepre-filling, each end of the tube is fitted with a stop valve connector25, 26. Pre-filling may be performed using the apparatus shown in FIG.12 and described below.

The coil of polymer tube 23, pre-filled with monomeric mixture, ispressurized during processing by pressure supply means 6 via pressuresupply line 7. Each end of pressure supply line 7 is fitted with a quickconnector that receive stop valve connectors 25, 26. Production of thepolymer light guide proceeds according to one of the examples givenabove, or a similar process. The apparatus shown in FIG. 10 has beenused to produce polymer light guides up to 500 metres in length.

A variation on the apparatus of FIG. 10 is shown in FIG. 11. In thiscase the coil of polymer tube 27 is kept on a drum 28. Typically, 500metre lengths of polymertube are commercially available on lightweight,cardboard drums. The drum 28 and tube 27 are located in a verticalchamber 29. As with the apparatus of FIG. 10, the ends of the tube 27exit the chamber and are fitted with quick connect stop valves 25, 26.The tube 27 may be pressurized during polymerisation by pressure supplymeans 6. A lid 30 may be fitted to the chamber.

In the embodiment of FIG. 11, temperature control in the chamber isachieved by circulation of temperature controlled air. A fan 31 drawsambient air through filter 32 into the chamber 29. The air exits thechamber through gaps between the lid and the body of the chamber. Exitvents can be formed in the lid 30 if required. A heater 33 is used toheat the air above ambient temperature. The heater 33 isthermostatically controlled to regulate the temperature in the chamber.

As mentioned above, the fluoropolymer tube may be pre-filled withmonomeric solution or filled in-situ in the reaction vessel or chamber.A suitable apparatus for filling the tube with monomeric mixture isshown in FIG. 12. A pressure vessel 40 is filled with a monomericmixture produced according to one of the recipes described above, or asimilar recipe. Pressure supply means 41 applies pressure to thepressure vessel 40 thereby pressurizing the monomeric mixture. Pressuresupply means 41 may suitably be an air compressor however a filter 42 isdesirable to avoid contamination of the monomeric mixture in thepressure vessel 41.

A coil of polymer tube 43 is connected to the pressure vessel 40 with aquick connect stop valve 44. A filter 45 may be fitted to the outlet ofthe pressure vessel 40 to remove any contaminants. The tap 46 is openedto commence filling of the tube 43.

The other end of the coil 43 is connected to a bleeding tank 47 withanother quick connect stop valve 48. Monomeric mixture is flowed throughthe coil and collected in the bleeding tank until any air in the tube iscleared. A vent 49 is open to atmosphere. When the tube has beensatisfactorily filled the vent 49 is closed.

The monomeric mixture in the tube can be pressurised by applyingpressure from pressure supply means 41. Pressure can be applied to bothends of the tube by applying pressure to the vent 49 in the bleedingtank 47. Once the tube has been disconnected from the filling apparatus,the monomeric mixture in the bleeding tank 47 can be collected fromdrain 50 and reused.

If necessary the filling procedure can be repeated a number of timesuntil the tube is well filled without voids, air bubbles orcontaminants.

The polymer light guides produced by the methods and apparatus describedabove provide good light transmission properties without furtherprocessing. If the optical entrance and exit faces become damaged orcontaminated, a new face can be formed by cutting the light guide usinga sharp knife. The inventors have found that a clean cut provides goodtransmission properties for most applications.

Improved transmission properties can be obtained by polishing theentrance and exit faces. A four step process has been found to be mosteffective. Initially a new face is cut, then a course wet and dry paperis used to flatten and square the face. A fine grade polishing slurry isused to smooth the surface by removing any scratches or imperfections.The face is buffed with a buffing pad until smooth and flat. Asurprisingly good quality surface is obtained for the polymer lightguide using this process.

The inventors have found that polymer light guides can be made in longlengths with good transmission properties using the methods andapparatus herein described.

Throughout the specification the aim has been to describe the preferredembodiments of the invention without limiting the invention to any oneembodiment or specific collection of features.

What is claimed is:
 1. A method of manufacturing a polymer light guideincluding the steps of: producing a monomeric mixture from amounts ofmonomer, initiator and cross-linking agent; forming a polymer tube intoa coil that is wound onto a drum, wherein the length of the polymer tubeis greater than 100 meters; filling the coil of polymer tube with themonomeric mixture; and pressurising and heating the full length of thepolymer tube to conditions appropriate to initiate and maintainpolymerisation of the mixture.
 2. The method of claim 1 wherein thelength of the polymer is greater than 300 metres.
 3. The method of claim1 further including the step of stirring the monomeric mixture to form auniform mixture.
 4. The method of claim 1 wherein the mixture is allowedto degas for a period of time.
 5. The method of claim 4 wherein theperiod of time is in the order of 30 minutes.
 6. The method of claim 1further including the step of purification of the monomer beforeproducing the monomeric mixture.
 7. The method of claim 6 whereinpurification is by vacuum distillation or ion exchange filtration. 8.The method of claim 1 further including the step of pre-expanding thepolymer tube by pressurising the tube prior to filling the tube with themonomeric mixture.
 9. The method of claim 1 wherein the step ofpressurising and heating the full length of the polymer tube isconducted in two or more steps In which the mixture in the polymer tubeis subjected to a first set of temperature and pressure conditionsfollowed by second and further sets of temperature and pressure.
 10. Themethod of claim 1 wherein conditions appropriate for initialisation ofpolymerisation include a pressure in the range 10 pa to 2000 kpa. 11.The method of claim 1 wherein conditions appropriate for maintainingpolymerisation include a temperature maintained in a range between thestorage temperature of the mixture and a temperature whereby the halflife of the initiator used is between 10 and 20 hours.
 12. The method ofclaim 11 wherein the temperature of the mixture is preferably increasedto a value whereby the half life of the Initiator used is between 5 and10 hours.
 13. The method of claim 11 wherein the temperature is furtherincreased to a value whereby the half life of the Initiator used sbetween 30 minutes and 80 minutes to obtain a desired final cure.
 14. Anapparatus suitable for manufacturing a polymer light guide, saidapparatus comprising: a coil of polymer tube supported by a drum whereinthe length of the polymer tube is greater than 100 meters; a reactionvessel for holding the coil of polymer tube, wherein the polymer tube isfillable with a monomeric mixture; pressure means connectable to thepolymer tube for applying pressure to the full length of said polymertube before and after filling said tube with said monomeric mixture; atemperature controlled fluid, circulable through the reaction vessel soas to control the temperature within the reaction vessel; a pump forcirculating the temperature controlled fluid; and temperature controlmeans for adjusting the temperature of the temperature controlled fluid.15. The apparatus of claim 14 wherein the polymer tube is completelyimmersed in the fluid within the reaction vessel.
 16. The apparatus ofclaim 14 wherein the reaction vessel is a room holding the drum.
 17. Theapparatus of claim 14 wherein the fluid is selected from water, oil orair.
 18. The apparatus of claim 14 further including a filling means.19. The apparatus of claim 18 wherein the filling means comprises apressure vessel containing a reservoir of monomeric mixture, a bleedingtank for receiving excess monomeric mixture and pressure supply meansfor supplying pressure to the pressure vessel to force monomeric mixturethrough the polymer tube when connected between the pressure vessel andthe bleeding tank.
 20. The apparatus of claim 14 further includingpolishing means for polishing the optical entrance and exit faces of thepolymer light guides produced in the apparatus.
 21. A method ofmanufacturing a polymer light guide of lengths greater than 100 metres,said polymer light guide having a polymeric core within a polymer tubehaving a lower refractive index than the polymer core, the methodincluding the steps of: producing a monomeric mixture from amounts ofmonomer, initiator and cross-linking agent; stirring the monomericmixture until a uniform monomeric mixture is obtained; filling a polymertube of greater than 100 metres length with the uniform monomericmixture, said polymer tube being wound on a drum so as to form a coil;placing the polymer tube filled with uniform monomeric mixture in areaction chamber at a first temperature; connecting the polymer tube toa pressure source and pressurising the full length of the polymer tubeto a first pressure; and subjecting the polymer tube to furthertemperature and pressure conditions, by changing the temperature in thereaction chamber and changing the pressure of the pressure source, untilthe uniform monomeric mixture has polymerised to a desired degree. 22.The method of claim 21 wherein said reaction chamber is maintained atambient pressure.